The present invention relates to logic comparators, and more particularly to tri-state logic comparators.
Prior art logic comparators compare the operation of a unit under test (UUT) with a known good, or reference, logic circuit or IC of the same configuration or part style as the UUT. To effect the comparison the same input stimulus is applied simultaneously to both the UUT and the reference logic circuit. These logic comparators typically include means for connecting to each of the external terminals of the UUT, comparator means for comparing the signals on the corresponding terminals of the UUT with those of the reference circuit, and interface means for presetting the comparator means for each terminal as a power, ground, input or output terminal. The interface means includes switches, specially designed program cards, (e.g., Hewlett-Packard Model 10529A), or programmable sockets (Fluke Model Trendar 200) to interconnect the corresponding power, ground and input terminals of the reference and UUT, and connects each of the corresponding output terminals to a separate exclusive OR gate where the signal from the UUT is compared with the signal from the reference circuit.
These types of comparators are primitive and cumbersome since they can only be used to test simple gates and counters. They do not provide repeatable results when testing a tri-state or open circuit output terminal of the UUT.
Additionally, these devices do not lend themselves to the testing of bi-directional devices where a single terminal of the device toggles between an input and an output function at such a rate so as to make it impossible to manually change the interface means as necessary during testing.
What is needed is a logic comparator which is capable of automatically discriminating between an input and an output terminal without the need for switches or program cards even when the change occurs during operation as with a bi-directional device, and one that can repeatably test a tri-state or open circuit output terminal, wherein it is only necessary to predesignate the power and ground terminals of the UUT and reference devices. The present invention provides such a device.